1. Field of the Invention
The present invention relates to an information embedding device, an information detecting device, an information embedding and detecting system, an information embedding method, an information detecting method, an information embedding program, an information detecting program, an information embedding and detecting program, and a recording medium. More particularly, the present invention relates to a technique of embedding information in a document image having a character region in such a manner that the information cannot be recognized by the human eye or is difficult to be recognized by the human eye, and a technique of detecting the embedded information.
2. Description of the Related Art
In recent years, techniques of embedding information in digital data such as image data and audio data have been developed in such a manner that the information cannot be recognized by human beings. By such techniques, copyright information can be embedded in data in advance, so that the owner of the copyright can be detected from a copy of the data. In this manner, unauthorized duplication of digital data that is provided over the Internet can be prevented. In the field of information embedding and detecting techniques to which the present invention relates, there are the following conventional techniques.
Japanese Laid-Open Patent Application No. 2000-99501 discloses “Method and System for Embedding Information in Document Data”. In this prior art, a method of embedding information in a document file such as a PDF or PS is disclosed. In accordance with this invention, a layout is analyzed, the characters located in a position designated by a pseudorandom number are extracted, and the relative location is controlled. Information embedding is then performed.
Japanese Laid-Open Patent Application No. 2001-78006 discloses “Method and Device for Embedding and Detecting Watermark Information in Black-and-White Binary Document Image”. In this prior art, information embedding and detection in a document image which is a binary image are disclosed. An image is divided into sub sections by a structural analysis conducted on the document image, and two or more sub sections form a group. Information embedding and detection are performed through comparison of feature quantities between groups. Japanese Laid-Open Patent Application No. 2002-232679 discloses “Image Processing Method and Apparatus, Computer Program, and Recording Medium”. In this prior art, information embedding is performed through quantization of the ratio of the sum of the gaps of neighboring characters to the difference between the gaps of the neighboring characters.
In accordance with each of the above inventions, each information embedded location is extracted with high precision through a structural analysis conducted on a document image. Redundant embedding locations are given to a set of information, so that the information detection can be performed with robustness. However, after the image in which information has been embedded is printed out, the information might be scanned and retrieved. In such a case, limitations are set on the amount of information that can be embedded and the stability in information detection. Here, there is also a problem that the reliability of the detected information cannot be measured.